1. Field of the Invention
The present invention relates to the field of solid-state chemistry, and, specifically, to the synthesis of mixed metal oxide ceramics, including perovskites.
2. Description of the Related Technology
Relaxor lead magnesium niobate Pb(Mg1/3Nb2/3)O3 (PMN) has been studied extensively because of its high dielectric constant and large electrostrictive coefficients. For example, 0.9PMN-0.1PT(PbTiO3) relaxor ferroelectric is a good candidate for multi-layer ceramic capacitor applications due to its high dielectric constant at room temperature. A 0.65PMN-0.35PT solid solution is a good piezoelectric material for sensor and actuator applications. However single-phase perovskite PMN could not be obtained by the conventional solid oxide method because of the presence of the pyrochlore phase, a reaction product between Nb2O5 and PbO. Swartz and Shrout first succeeded in eliminating the pyrochlore phase by developing the columbite method that involved two calcination steps:
                                          MgO            +                                          Nb                2                            ⁢                              O                5                                              ⁢                      →                          1000              ⁢              °              ⁢                                                          ⁢                              C                .                                              ⁢                                    MgNb              2                        ⁢                          O              6                                      ,                                  ⁢        and                            (        1        )                                                                    MgNb              2                        ⁢                          O              6                                +                      3            ⁢            PbO                          ⁢                  →                      700            -                          900              ⁢              °              ⁢                                                          ⁢                              C                .                                                    ⁢                  3          ⁢                      Pb            ⁡                          (                                                Mg                  113                                ⁢                                  Nb                  213                                            )                                ⁢                                    O              3                        .                                              (        2        )            In the first calcination step, mixtures of Nb2O5 and MgO were heated near 1000° C. to form the columbite phase, MgNb2O6. In the second calcination step, MgNb2O6 was mixed with PbO and heat-treated. The perovskite phase began to appear near 700° C. and complete perovskite conversion occurred near 900° C. The two calcination temperatures may vary with a number of parameters such as reactivity of MgO, degree of mixing, and control of the PbO volatility. Nevertheless, two calcination steps were needed to prevent the direct contact between Nb2O5 and PbO and, thus, the formation of the pyrochlore.
Other methods including sol-gel methods solution processes, soft mechanochemical pulverization, co-precipitation, thermal spray, and Mg(NO3)2 mixing have also been developed to prepare pyrochlore-free PMN-PT powders. These methods were based on the principle of improving the reactivity of MgO either by optimizing the powder characteristics including particle size, specific surface area, reactivity of raw materials such as Mg(OH)2 or Mg(NO3)2, or by using a high-energy milling method. A molten salt method has also been shown to produce single-phase perovskite PMN-PT powders. Of all the available methods, the columbite method is still the most widely used method in preparing lead-based relaxor ferroelectrics because of its less stringent requirements on the raw materials and the high reliability of the process. However, the columbite method requires two calcination steps, i.e., the formation of the columbite phase at around 1000° C. followed by the complete formation of the perovskite phase at 900° C.
Because of the presence of pyrochlore phase, PMN ceramics are sintered using perovskite PMN powders. The regular sintering temperature of PMN ceramics is around 1200° C. At this temperature, the lead loss is serious. This results in an imprecise composition and deterioration of the final properties. In addition, with such a high sintering temperature low-cost electrodes such as Ag and Cu cannot be used to produce multi-layer capacitors and multi-layer actuators.
Another disadvantage of the traditional columbite method is the requirement of multiple processing steps. The traditional columbite method requires three ball milling steps, two calcination steps and one sintering step to obtain the final PMN ceramics. A process employing all of these steps is cost-prohibitive.
Although PMN-PT solid solutions possesses the best properties among its class, the commonly used technology for making PMN-PT solid solutions requires multiple heat treatment steps and a temperature of 1200° C. for the final sintering step. The cost associated with the multiple processing steps and the cost of special electrode materials that can sustain the high sintering temperature make PMN-PT solutions uncompetitive in important applications such as for multi-layer capacitors and multi-layer actuators. Some methods were found to permit lower sintering temperatures. For example, by adding 5-21 wt % excess of PbO, the sintering temperature can be reduced to 950° C. Adding 1-4 at % of SrO permits use of a sintering temperature as low as 800° C.-900° C. Use of PMN powder made by the Mg(NO3)2 mixing method, allows the sintering temperature to be reduced to 900° C. These methods, however, only permit lowering of the sintering temperature, but they still require multiple processing steps that result in a prohibitively high production cost.
By directly compacting the columbite phase and PbO into a green-body and sintering, the columbite method could be reduced to requiring two ball milling steps, one calcinations step, and one sintering step. However, the sintering temperature had to be increased to 1250° C. Later, with the same method, the sintering temperature was able to be reduced to 1000° C. by sintering a mixture of very fine TiO2 powder with more reactive (PbCO3)2Pb(OH)2 in an O2 atmosphere. Although this method had the advantages that it required fewer processing steps than the columbite method, and that it lowered the sintering temperature to 1000° C., this process still required two ball-milling steps, one calcination step and one sintering step. In addition, this method suffers from the additional disadvantages that there are additional costs associated with the fabrication of the required nanosize TiO2 powder and for the provision of the O2 sintering atmosphere, which still makes this method cost-prohibitive.
U.S. Pat. No. 5,079,199 (Ochi et al.) proposes to solve the problem of liquid Pb2WO5 formation during the production of lead magnesium tungstate by first reacting MgO with WO3 to form MgWO4, mixing, pressing and reacting with PbO to form the desired product. In this manner, liquid Pb2WO5 formation is prevented. Ochi et al. filters, dries and reacts the mixture of MgO with WO3 at 750-1000° C. to form magnesium tungstate powder. Mixtures of the magnesium tungstate powder were made with PbO, NiO, Nb2O5, MgO and TiO2 in a ball mill. Each mixture was then filtered, dried, and calcined at 750-850° C., a disk was made and the disk was sintered in air at 925-1050° C. for one hour. Ochi et al. also mentions that a similar process can be employed for the manufacture of ceramic compositions containing as the main component, one or more perovskite compounds such as lead magnesium niobate.
In summary, each of these methods either reduced the complexity of the columbite process or reduced the required sintering temperature. However, none of these methods solved both the cost problem and the problem that sintering must be carried out at a high sintering temperature.
Therefore, there remains a need for an improved and cost effective method for making perovskites, as well as other mixed metal oxide ceramics in order to make these materials competitive in the market place. Although the above methods address certain aspects of making perovskites, none have managed to create a truly cost effective method of making perovskites. Applicants have managed to create such a method by transforming the pyrochlore phase commonly found in perovskite production. In transforming this phase Applicants have been able to reduce the production of perovskite to a single-step, low temperature reactive sintering method. This method is also broadly applicable to the synthesis of other mixed metal oxide ceramics and need not be used solely for perovskites.